Turbo boost build up without increase RPM...

Hello~

I just bought a Suabru Legacy 2.0 (with 5 manually gear), it is a sport sedan with turbocharger engine. The car come with a turbo timer for display the boost value and control the turbo cooling time before shut down the engine. I drive my car softly most of the time. For most of the highway, I will keep the car with around 2000rpm at 5th gear to get a 80km/h steady cruise speed, even it is going up-to-the-slope.

Last week when I was driving, I keep the speed at 80km/h @ 2000rpm while going up-slope on a highway, then I wanna accelerate to higher speed during the slope, I just step fully down the throttle. Suddenly I feel the acceleration is "choking"! Just feel like the compressor is going "spinning and slow down" again and again, then the turbo timer alaram with a warning signal. From the reading of the timer, the boost value goes up to 1.3kg/cm2. I know this must be the pre-set limit of boost value to protect the compressor. In normal day-time driving, the boost value always around -5X to 0.x kg/cm2.

According to this condition, I got several question

1. Why the boost will build up to so high level when I step down the throttle fully? Though I step down the throttle fully at the mean time, the engine speed haven't build up quickly, the exhausted gas should not have a big change in flow to make the turbine spin fast which generate much more boost.

2. For the timer the defaul boost limit is around 1.1-1.2kg/cm2, will it a too hidebound or boldness value? How much will be this boost in converting to BAR unit? If I never install this turbo timer, I will never know that it's overboost in this situtation, which will damage the engine, isn't it?

3. In steady speed driving on flat road, the boost value mostly between -40 to -50 kg/cm2, why the pressure will be in negative value?

4. From some of the information from internet, it said the turbine is start spinning when the engine is started, is it true? In my knowledge, if the compressor make the boost reaching limit, pressure will trigger the wastegate to let exhaust gas bypass the turbine and balance the pressure. So if I drive the car in low RPM (cruise on highway at 2000rpm, 80km), the compressor will keep charging and build up pressure, doesn't it will damage the comprees under long time?

Thanks for reading~

1. Why the boost will build up to so high level when I step down the throttle fully? Though I step down the throttle fully at the mean time, the engine speed haven't build up quickly, the exhausted gas should not have a big change in flow to make the turbine spin fast which generate much more boost.
When you are at cruising speed, your at 15-20% throttle. When you go to 100% throttle, you are adding 5 or 6 times more air and fuel. That will have a huge effect on turbo speed even though the RPMs don't go up.

In steady speed driving on flat road, the boost value mostly between -40 to -50 kg/cm2, why the pressure will be in negative value?
A car without a turbo operates in negative pressures all the time. vacuum is greatest at idle and deceleration and least at full throttle. With a turbo, the same is true, except you get positive pressure when the turbo makes more pressure than the engine is taking in. On a flat road you are using very little throttle, so the turbo isn't making much pressure. The engine is sucking more air in than the the turbo makes, therefore you have negative pressure.

4. From some of the information from internet, it said the turbine is start spinning when the engine is started, is it true? In my knowledge, if the compressor make the boost reaching limit, pressure will trigger the wastegate to let exhaust gas bypass the turbine and balance the pressure. So if I drive the car in low RPM (cruise on highway at 2000rpm, 80km), the compressor will keep charging and build up pressure, doesn't it will damage the comprees under long time?


The turbine will start spinning when the engine starts, but not fast at all. It won't be making pressure. Its not about building up pressure and then letting it go, it makes pressure constantly. Think of it like this example: Put a small drinking straw in your mouth and blow through it. The blowing is like your exhaust. Now blow harder. You increase the pressure in your mouth because of the restriction in the straw. Now blow harder. Once you reach a certain amount of pressure in your mouth, it will start "leaking" past your lips. You won't be able to keep your lips closed against the pressure. That's how a wastegate works. After you reach a certain pressure, it starts letting it leak past. It doesn't "build up" pressure, its a constant balance of how much exhaust goes past the turbo and how much is let out the other side.

Have you ever blown on a fan and made it spin? That's how a turbo works. Blow harder and the fan spins faster. Its just that the fan is attached to a compressor which pushes intake air.

When you are at cruising speed, your at 15-20% throttle. When you go to 100% throttle, you are adding 5 or 6 times more air and fuel. That will have a huge effect on turbo speed even though the RPMs don't go up.

So how will adding 5 or 6 time of air and fuel affect the turbo speed under "RPM don't go up" condition?

As you say "its a constant balance of how much exhaust goes past the turbo and how much is let out the other side." I don't understand how pressure goes up without more exhaust gas to drive the turbine...


A car without a turbo operates in negative pressures all the time. vacuum is greatest at idle and deceleration and least at full throttle. With a turbo, the same is true, except you get positive pressure when the turbo makes more pressure than the engine is taking in. On a flat road you are using very little throttle, so the turbo isn't making much pressure. The engine is sucking more air in than the the turbo makes, therefore you have negative pressure.

Got it, that's mean the turbo is building pressure all the time, start from engine ignited. The negtaive value is caused by the "Suction is more than generate"



The turbine will start spinning when the engine starts, but not fast at all. It won't be making pressure. Its not about building up pressure and then letting it go, it makes pressure constantly. ....

So that's mean the wastegate is in work (releasing the pressure build by compressor) when the engine is started, is it?

Thanks for your reply :loser:

So how will adding 5 or 6 time of air and fuel affect the turbo speed under "RPM don't go up" condition?

Because you are multiplying the volume of exhaust by 5 or 6. You are pushing 6 times more exhaust through the tubine than you are at light throttle. The speed of the exhaust gasses are only part of what makes the turbo spin. The main factor in making the turbine spin is how much mass is flowing through it. Sometime try an experiment. Accelerate to 70 mph in 5th gear. Now floor it for a few seconds. Notice how the boost pressure goes up. That is because you have given the turbine a whole lot more exhuast gas to move it. Now take your foot off the pedal. Notice how the pressure goes negative. That's because you have closed the throttle and the turbo has almost no exhaust flow to make it run anymore and the engine is still sucking air.

As you say "its a constant balance of how much exhaust goes past the turbo and how much is let out the other side." I don't understand how pressure goes up without more exhaust gas to drive the turbine...

When you floor the pedal even though RPMs don't go up, you are putting A LOT more exhaust through the turbo.



Got it, that's mean the turbo is building pressure all the time, start from engine ignited. The negtaive value is caused by the "Suction is more than generate"


So that's mean the wastegate is in work (releasing the pressure build by compressor) when the engine is started, is it?

No, the turbo is not building pressure all the time. When you start the engine, the small flow of exhaust makes the turbo spin just a little bit. You could easily stop it with your finger. Its not making any pressure.

The wastegate only works when you are at full boost to prevent too much pressure from entering the intake. The way it works is this: The exhaust flow blows on a fan (the turbine). The turbine connects to another fan (the compressor) that blows extra air into the intake. So, the only time the compressor is making pressure is when you are making enough exhaust flow to push the turbine really hard. If you hold your foot to the floor, the turbo will make more and more pressure to the point where the wastegate has to open and prevent too much pressure.

Because you are multiplying the volume of exhaust by 5 or 6. You are pushing 6 times more exhaust through the tubine than you are at light throttle. The speed of the exhaust gasses are only part of what makes the turbo spin. The main factor in making the turbine spin is how much mass is flowing through it. Sometime try an experiment. Accelerate to 70 mph in 5th gear. Now floor it for a few seconds. Notice how the boost pressure goes up. That is because you have given the turbine a whole lot more exhuast gas to move it. Now take your foot off the pedal. Notice how the pressure goes negative. That's because you have closed the throttle and the turbo has almost no exhaust flow to make it run anymore and the engine is still sucking air.

From your words, is it containing a theory: "Increasing in exhaust gas quantity has NO ABSOLUTE relationship with RPM goes up"?

Or, there is another situtation - "There is a valve controlling how much exhausted gas flow to the turbine"?



When you floor the pedal even though RPMs don't go up, you are putting A LOT more exhaust through the turbo.

That's the point:
"Keep steady RPM = Exhaust same amout of gas (even floor pedal)"?

From your words, is it containing a theory: "Increasing in exhaust gas quantity has NO ABSOLUTE relationship with RPM goes up"?

You can increase exhaust quantity two ways; more throttle, or more rpms. If you keep the same throttle and downshift so you're at higher rpms, there will be a little more exhaust. The throttle hasn't opened any more, but the engine is sucking harder and sucking more air in, so it's putting more exhaust out. If you push the acclerator more (open the throttles more) you are letting in a lot more air even though the RPMs might not go up. More air in means more exhaust out. Both situations will cause the turbo to spin faster and make more pressure.

Or, there is another situtation - "There is a valve controlling how much exhausted gas flow to the turbine"?

That situation only happens when peak boost is reached. If the engine is tuned for 10 psi, 11 psi might cause damage. So, when the intake reaches 10 psi, the wastegate starts to open and let exhaust go around the turbine instead of through it. 90% of the time, the wastegate is closed and all the exhaust goes through the turbine. Only when you're really acclerating hard will the wastegate operate.

That's the point:
"Keep steady RPM = Exhaust same amout of gas (even floor pedal)"?

No... steady rpm + floor pedal = more exhaust gas

Thanks so much Mr. Curtis73, I still got one silly question according to this turbo concept:

"If you push the acclerator more (open the throttles more) you are letting in a lot more air even though the RPMs might not go up. More air in means more exhaust out."

In my imagination, only RPM goes up (engine run faster) can figure out the increase of exhaust gas. But really can't figure out how the situtaion of "Engine suck and exhaust more gas but keep its running speed"? It sucks more fresh air, consume more fuel, exhaust more gas, should have more energy come out. But now it doesn't , it just stay the same speed. why?

Thanks so much Mr. Curtis73, I still got one silly question according to this turbo concept:

"If you push the acclerator more (open the throttles more) you are letting in a lot more air even though the RPMs might not go up. More air in means more exhaust out."

In my imagination, only RPM goes up (engine run faster) can figure out the increase of exhaust gas. But really can't figure out how the situtaion of "Engine suck and exhaust more gas but keep its running speed"? It sucks more fresh air, consume more fuel, exhaust more gas, should have more energy come out. But now it doesn't , it just stay the same speed. why?

For an engine, air in through the throttle has to come out the exhaust.
At part throttle you're restricting the air in, which restricts the amount of exhaust out.
When you open the throttle (at any rpm) the amount of air into the engine greatly increases, this produces more exhaust which drives the turbo to produce even more intake air (boost).

Increasing rpm also increases the airflow (if you're comparing similar throttle position).

The question I just mention, according from Curtis reply, if floor flat the pedal, large amount of air go into engine, more fuel will consume, more gas will exhaust, it should produce more energy, but, now the engine no increase in speed. So, where are those energy goes to? I know this situtation is not only happen in a turbo charge engine, but even a normal absorbtion engine...

And then, in my case (the up-slope floor padel without RPM increase but boost), the wastegate should be triggered if i shut down the turbo timer, is it? I just wonder is it the boost limit in my turbo timer setting is much lower than the car's wastegate working pressure....

Don't they make electrical wastegates? If so, turbotimer is probably the only controller for the wastegate.

Don't they make electrical wastegates? If so, turbotimer is probably the only controller for the wastegate.

I don't think so, because the original car have no turbo timer, it is just an retail part for Legacy

Thanks so much Mr. Curtis73, I still got one silly question according to this turbo concept:

"If you push the acclerator more (open the throttles more) you are letting in a lot more air even though the RPMs might not go up. More air in means more exhaust out."

In my imagination, only RPM goes up (engine run faster) can figure out the increase of exhaust gas. But really can't figure out how the situtaion of "Engine suck and exhaust more gas but keep its running speed"? It sucks more fresh air, consume more fuel, exhaust more gas, should have more energy come out. But now it doesn't , it just stay the same speed. why?

In your original example, you talked about going up a hill. That is a good example of more throttle with the same RPMs. The extra energy you produce goes to overcome gravity.

But, you could picture it another way. Put it in 5th gear while going 40 mph. Then floor the pedal. RPMs might rise, but very slowly. But, you are still increasing the exhaust flow a lot. Air in = air out. The extra air you're letting in with the open throttle has to go out the exhaust. In this situation, you're not making enough extra energy to raise RPMs very fast, but the turbo will be spinning faster even though RPMs didn't rise much at all.

icic.... so in my case, the wastegate should be triggered if i shut down the turbo timer, is it? I just wonder is it the boost limit in my turbo timer setting is much lower than the car's wastegate working pressure....

Thanks mr curtis, u help me to solve a big big problem which have no answer within those car forum in my country! Thank you very much!


A little question, in rally car (like WRX and EVO) racing in WRC, there always a "bird whistle" like blow off valve sound come out from those car. What's this type of BOV exactly? I have heard many brands of BOV (HKS, Blitz, Sard...) the sound are absolute not in this kind.

turbo timers are usually just components that let the car run with the doors locked, for a designated time to allow the turbo to cool off after a hard drive. The theory is to prevent the turbo from getting coked up

They don't normally have any connection with the engine, turbo or anything else under the hood. Just the ignition switch. You might have something that has other components in addition to its base function though.

Oh~ my turbo timer is not just only a timer, but it can show the speed and boost and give out warning signal if there reaching the limits, i just wonder if it connect to my car computer to cut the fuel if the boost really reaching limits...

A little question, in rally car (like WRX and EVO) racing in WRC, there always a "bird whistle" like blow off valve sound come out from those car. What's this type of BOV exactly? I have heard many brands of BOV (HKS, Blitz, Sard...) the sound are absolute not in this kind.

A BOV is a secondary way of controlling maximum pressure. If you are at maximum boost (wide open throttle) and then go to no throttle, the turbo pressure spikes (gets really high) because the turbo is at max effort and you shut off its only outlet. The pressure spikes can cause the turbo to slow down alot with all that extra pressure. The BOV leaks extra pressure and allows the turbo to stay spinning faster, so its ready for boost the next time you open the throttle.

It also acts as a safety. If the wastegate fails, it could over-boost the engine and cause damage. The BOV will leak off extra pressure and prevent damage.

On the street, a BOV is not necessary.

To Curtis:

in my case, the wastegate should be triggered if i shut down the turbo timer, is it? I just wonder is it the boost limit in my turbo timer setting is much lower
than the car's wastegate working pressure....

I don't know if this was discussed earlier because I didn't get a chance to read the whole thread, even though it sounds very interesting. Turbo chargers aren't all designed EXACTLY alike. The turbo I saw on my friend's porsche 911 only spins up after the throttle is past a certain position. So the turbo doesn't engage all the time. The exhaust moves freely without spinning up the turbine. So I would assume that his turbo has some form of a transmission that disengage the turbo at low throttle. Others have a waste gate which is purely mechanical and the turbo is providing boost at all times except when the turbo starts providing too much pressure in which the waste gate lets excessive air out. Right?

I'm going to copy this thread over to our forced induction forum where it might get some better responses.

I don't know if this was discussed earlier because I didn't get a chance to read the whole thread, even though it sounds very interesting. Turbo chargers aren't all designed EXACTLY alike. The turbo I saw on my friend's porsche 911 only spins up after the throttle is past a certain position. So the turbo doesn't engage all the time. The exhaust moves freely without spinning up the turbine. So I would assume that his turbo has some form of a transmission that disengage the turbo at low throttle. Others have a waste gate which is purely mechanical and the turbo is providing boost at all times except when the turbo starts providing too much pressure in which the waste gate lets excessive air out. Right?

I'd bet the porsche simply has a larger turbo that requires more airflow to get it pumping air.
Below the airflow needed, it'll basically just idle around, providing a small restriction to airflow but producing no measurable boost.

Yeah I don't know much about turbos but I do know that my buddy's '86 Porsche 911 is notorious for turbo lag. I'm not really sure why the turbo doesn't spin up from regular driving, but it really only kicks in when the peddle is basically on the floor. He once told me "your steering wheel better be pointing straight when the turbo kicks in, or you're dead!".

Yeah I don't know much about turbos but I do know that my buddy's '86 Porsche 911 is notorious for turbo lag. I'm not really sure why the turbo doesn't spin up from regular driving, but it really only kicks in when the peddle is basically on the floor. He once told me "your steering wheel better be pointing straight when the turbo kicks in, or you're dead!".

Older air-cooled porsches needed to use bigger turbos. Smaller turbos which create lots of low-end boost make screaming hot pressures at higher RPMs. In an air cooled engine, that's no good. Since its a race-derived car anyway, they opted for the larger turbo that creates boost at higher RPMs to be more compatible with the air cooled engine.

Compare that with the Chrysler fashion turbos of the 80s. They were tiny turbos that produced copious boost at low RPMs to make the driver feel that instant boost, but they choked off high RPM flow and made hot, inefficient air. Since they were water cooled, they could take the heat, but they were designed to fulfill a buzzword market niche.

And, not to nitpick... but don't confuse boost threshold with turbo lag. Turbo lag is how long it takes the exhaust to overcome the inertia of the turbine. Basically it means how long it takes to get the turbo spinning after teh engine is producing enough exhaust to move it. Boost threshold is basically the time it takes from when you mash the pedal until when boost starts being produced. Porsche engineered their cars with a high boost threshold, but that shoudn't be labeled at turbo lag.

And, not to nitpick... but don't confuse boost threshold with turbo lag.

That's a pet peeve of mine too.:cool:

For an engine, air in through the throttle has to come out the exhaust.
At part throttle you're restricting the air in, which restricts the amount of exhaust out.
When you open the throttle (at any rpm) the amount of air into the engine greatly increases, this produces more exhaust which drives the turbo to produce even more intake air (boost).

Increasing rpm also increases the airflow (if you're comparing similar throttle position).

To Mr Kiwi and Curtis,

so if the RPM is low and I open the throttle big, lots of air intake, lots of exhaust will generated and drive the turbo to build up pressure, but doesn't the big throttle and the engine can consume this pressure and balance at this "lots of air intake" condition??
how about in high RPM? can the pressure generated by turbo fully consume by the engine?

To Mr Kiwi and Curtis,

so if the RPM is low and I open the throttle big, lots of air intake, lots of exhaust will generated and drive the turbo to build up pressure, but doesn't the big throttle and the engine can consume this pressure and balance at this "lots of air intake" condition??
how about in high RPM? can the pressure generated by turbo fully consume by the engine?

At low speeds the engine will consume all the turbos air without creating any boost. But once you reach the airlflow that the turbo needs to create boost, it will pump more air, resulting in higher pressure (boost) in the intake system.

You can feel this point quite well when driving a turbo car.